I put a couple of epubs from the Gutenberg site onto my PRS-T1. In both cases the book opens OK but I am unable to change the size of the font (Original) and if I select a different font type the book automatically closes, returning me to the home page.

It's not common with Gutenberg book files but you do find it from time to time. Easiest path is to use Sigil or Calibre ("Tweak Epub" functionality) and delete the font-size declaration from the CSS file.